A back-illuminated image sensor having the light receiving face of a photodiode on the back surface of a semiconductor substrate has attracted attention in the field of solid-state imaging devices such as a CMOS (Complementary Metal Oxide Silicon) sensor and a CCD (Charged Coupled Device). The back-illuminated image sensor, which does not require to form wiring and an extra film on the light receiving face, can obtain a sensitivity higher than that of a front-illuminated image sensor.
When manufacturing the back-illuminated image sensor, a first wafer having photodiodes etc. formed thereon is bonded to a second wafer, and then the back surface of the first wafer is polished to be thinned. After that, the back surface of the second wafer is also polished to be thinned. At the point when starting polishing the second wafer, the first wafer has already been thinned, and the surface on its periphery has gotten rough due to the polishing. This rough surface causes the film having photodiodes etc. to come unstuck, and a polishing dust lowers film quality.
Accordingly, it is considered to form a smooth recess along the periphery of the first wafer before polishing the second wafer.
When polishing the second wafer, a protective tape is attached to the surface of the first wafer having the recess formed thereon, and then the back surface of the second wafer is polished. Since the bonding film of the protective tape does not come into contact with the whole surface of the recess, the polishing dust enters the gap between the bonding film and the surface of the recess, which leads to an increased incidence of deficiencies such as separation of the bonding film and reduction in film quality. Further, the recess having a wide width reduces the area to form elements on the wafer, which deteriorates productive efficiency. Further, when the recess has an excessively deep depth, the periphery of the wafer may possibly chip.